Bandwidth reduction within packet switched networks by not sending idle timeslots

ABSTRACT

The present invention describes a method in telecommunication networks where time division multiplexing traffic is transported over packet switched networks comprising one or a number of transmitting parties where the transmitting party will not send idle timeslot data on the transporting network. In one preferred embodiment will the transmitting party provide information regarding which timeslot hat are not used. Said telecommunication network can preferably be a packet switched network, wherein the packet switched network is one of the following: an IP-network, MPLS, ATM or Frame Relay.

FIELD OF THE INVENTION

The present invention is related to telecommunication networks and inparticular to methods where time division multiplexing traffic istransported over packet switched networks comprising one or a number oftransmitting parties, and one or a plurality of receiving parties.

BACKGROUND OF THE INVENTION

The invention is applicable where the Time Division Multiplexingtechnique (TDM) is used on the site and a packet based protocol, i.e.the Internet Protocol (IP), is implemented in the transport networkbetween the sites. Traditionally TDM (Time Division Multiplexing)traffic is today transmitted directly over SDH (Synchronous DigitalHierarchy) networks. However, it is assumed that tomorrow's telecomtransport network will be packet based, but probably with a long reusetime of existing site equipment.

Many vendors believe that the future network protocol will be theInternet Protocol (IP). With the introduction of this network technologyfor real-time applications, telecom companies may get a dilemma. On onehand they want to adapt to the new technology, on the other hand theyhave a lot of equipment like MSC (Mobile Switching Centre); BSC (BaseStation Controller) etc. working very well with circuit switchedtechnology. Buying a complete new packet based solution right away andto phase out circuit switched equipment will be very expensive. Anotherpossibility is to maintain circuit switched and packed switched networksin parallel, but this may also be regarded as too expensive. Facing thefact that the operator on long term probably will implement packet-basedtransport solutions anyway, i.e. the Internet Protocol (IP), a wantedsolution is a mechanism that allows the Telecom Company to use today'sequipment such as MSCs, BSCs etc. towards a packet switched network asan intermediate solution. Then in a later step, when the packet basedtechnology is considered mature for real time applications on site, thechange to an “all packet switched” scenario could be regardedreasonable. A problem when transporting TDM over packet based networksis the bandwidth utilization in the transport network. With TimeDivision Multiplexing, the connections are separated in timeslots.Depending on the traffic load situation, there will be a variablenumbers of timeslots not carrying any traffic (they are IDLE). Eventhough there are timeslots not carrying traffic, these timeslots aretoday filled with a so-called “Idle Pattern”, “Idle pattern” is a fixedbit pattern and it is used in synchronous systems where there can not beany “holes” in the data stream, and transmitted through the network.

A draft to the Internet Engineering Task Force (IETF) describing theconcept TDM over IP has been posted. (TDM over IP, Yaakov (Jonathan)Stein et al. March 2003) In this draft there is also a suggestion as towhat the protocol stack could look like. The mentioned drafts have nooption for dynamic IP bandwidth reduction. There are methods fortransporting parts of a trunk (24/32 timeslots), but in a more staticway. The described method is rather complicated, introducing a lot ofextra overhead etc.

This invention describes a solution to avoid the transmission of thetimeslots carrying Idle Pattern and hence reducing the average size ofIP packets transmitted. The IP bandwidth reduction requires thatinformation is being sent from the transmitting party to the receivingparty, about which timeslots are, at a given moment, not carryingtraffic. Equipment existing today provides information about whether atimeslot contains traffic or not.

The present invention uses this information to avoid transmittingtimeslots not carrying traffic.

The invention introduces a method for reducing the total amount of datatransmitted in a system where traditional TDM circuit switched data aretransported over a packet switched network. This will bring a betterutilization of the network resources.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a system/device andmethod that eliminates the drawbacks described above. The featuresdefined in the claims enclosed characterize this method.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to make the invention more readily understandable, thediscussion that follows will refer to the accompanying drawings.

FIG. 1 shows an example where the invention is applied.

FIG. 2 shows the organization of the transmitted data in ETSI 32 chstandard.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The major achievement for the present invention is the reduced need forbandwidth, due to the fact that a transmitting party will not send idletimeslot data on the transportation network, but only signal to thereceiving party which timeslots are not used, hence there will beincreased bandwidth utilization in the transport network.

In one embodiment of the invention, as depicted in FIG. 1, a scenariowith TDMoIP is shown. At MSC A, traditional TDM traffic comes from theGroup Switch. Today this traffic goes to an Exchange Terminal (ET)before it is sent out on the SDH network. The invention uses the“TDMoIP” Exchange terminal, which can encapsulate TDM traffic into IPpackets. From TDMoIP the data is sent to the packet router R1communicating with MSC A. Through the IP network, MSC A can communicatewith both MSC B and MSC C which are connected through routers R4 and R2respectively. The invention requires that the TDMoIP units receivesignalling information on which timeslots are idle. In FIG. 1, both MSCA and MSC C have to communicate with MSC B through router R3. If theinvention is used, it could in this scenario i.e. ease the work forrouter R3, avoiding queue and dropped packets due to fewer/shorter IPpackets. In general the invention is particularly useful for theintermediate routers, and the more hops the more favourable is theinvention.

The principle of the packet based, i.e. IP, bandwidth reduction is thatthe transmitting party will signal which timeslots are not carrying anytraffic. These timeslots would, without this invention, carry the “Idlepattern” and hence block this capacity for other users. The idea is toremove the “idle pattern” data from the payload and only signal thatthis timeslot is not carrying any user traffic. Arriving at thedestination in the transport network, the packet-based bearer will beterminated. The incoming data will, upon some signalling bits, find outwhich timeslots are not transmitted. The receiving party will theninsert the “Idle pattern” into the data stream at the right position.This will happen at the point where i.e. the Internet Protocol (IP) isterminated. The original circuit switched data are then reproduced.

To reproduce the original data stream, the receiving site will insertthe “Idle Pattern” data into the stream based on the signalling from thetransmitting party. Information about which timeslots are transmitting“Idle Pattern” data is i.e. available as an “Idle Pattern Flag” in theDL34 interface from the Ericsson Group Switch on site. It must be notedthat according to the standards, the transmitting party cannot removetimeslots from the data stream based on analyses of the data streamitself because the transmitted data could accidentally be identical tothe “Idle Pattern”. The unused timeslots should be removed based oninformation from the signalling in the system (i.e. from Group Switch orISUP).

The signalling part can be solved in different ways depending on thestandard used. The 32-channel ETSI (European Telecommunication StandardsInstitute) standard, timeslot 0 (TS-0) contains some free bits, markedas Sa in the FIG 3. It is also possible to use other bits in TS-0 asmany of them have fixed values not representing any information value(sync), as it is known that the data arrive when the IP packet hasarrived. For this standard, TS0 will always have to be present in thedata stream.

For the 32-channel solution depicted in FIG. 2, 31 bits are needed tosignal which timeslots are idle, it is not necessary to signal for TS-0,as this timeslot is always used. Table 1 shows the standardized contentof timeslot 0. There are 20 free bits (Sa-bits) in one sub multi-frame.In order to signal for 31 timeslots, 2 sub multi-frames are needed. Thismeans that Sa4 in Frame 1 in Sub multi-frame 1 will i.e. be used tosignal whether timeslot 1 is idle or not. Sa5 in Frame 1 will i.e. beused to signal whether timeslot 2 is idle or not etc. As one cannotsignal for all timeslots in every frame, there could be somedelay/losses before the far end notices that a given timeslot isidle/not idle anymore. The exact number will be 16 frames, each of 125microsecond's duration, totally giving 2 milliseconds delay. Eachconnection (timeslot) will, theoretically in a “zero delay” system,loose 16 bytes of data. This is normally not critical neither for speechnor for data, as the higher network end-to-end signalling protocols areslower than the process turning Idle Pattern on/off. If it is importantnot to loose any frames with valid data, it can be implemented with acheck for frames containing valid data for a certain timeslot, timeslotswith valid data can then be marked as “not idle” before the multi-frameis sent. The drawback is that a delay of 1 ms will be added with such asolution. Sub Multi Bit 1 to 8 in Timeslot 0 Frame Frame 1 2 3 4 5 6 7 81 0 C1 0 0 1 1 0 1 1 1 0 1 A Sa4 Sa5 Sa6 Sa7 Sa8 2 C2 0 0 1 1 0 1 1 3 01 A Sa4 Sa5 Sa6 Sa7 Sa8 4 C3 0 0 1 1 1 1 1 5 0 1 A Sa4 Sa5 Sa6 Sa7 Sa8 6C4 0 0 1 1 1 1 1 7 0 1 A Sa4 Sa5 Sa6 Sa7 Sa8 2 0 C1 0 0 1 1 0 1 1 1 0 1A Sa4 Sa5 Sa6 Sa7 Sa8 2 C2 0 0 1 1 0 1 1 3 0 1 A Sa4 Sa5 Sa6 Sa7 Sa8 4C3 0 0 1 1 1 1 1 5 0 1 A Sa4 Sa5 Sa6 Sa7 Sa8 6 C4 0 0 1 1 1 1 1 7 0 1 ASa4 Sa5 Sa6 Sa7 Sa8

This opens for another way of signalling which timeslots are IDLE. Ifthis new protocol shall be able to handle the bandwidth reductionsolution described in this document, some bits must be reserved for thispurpose. In the long term this will probably be the best solution and itwill be usable for both 24 channel and 32 channel systems.

The invention can be used for all kinds of traffic based on TDMtechnology that needs to be transported through a packet-based network.Examples of packet based transport networks are IP, MPLS, ATM, and FrameRelay etc. To be able to use the bandwidth reduction, there must be someindication (i.e. from the Group Switch or other signalling) available asto which timeslots are filled with Idle Pattern.

It is also possible to add an extra field in the protocols used forsignalling which timeslots are filled with Idle Pattern. The drawback isthat whenever there are no idle timeslots, the packet size will beincreased due to the introduction of the extra signalling bits.

This principle could be used for any known fixed bit pattern, not onlyfor the Idle Pattern used in the standard TDM telephone systems.

List of Abbreviations

-   ATM Asynchronous Transfer Mode-   BSC Base Station Controller-   ETSI European Telecommunication Standards Institute-   IETF Internet Engineering Task Force-   IP Internet Protocol-   ISUP ISDN User Part-   MPLS Multi Protocol Label Switching-   MSC Mobile Switching Centre-   RFC Request for Comments-   SDH Synchronous Digital Hierarchy-   TDM Time Division Multiplex-   PSN Packet Switched Network

1-11. (canceled)
 12. A method for transporting time division multiplexedtraffic over packet switched networks between transmitting partiescomprising the steps of: a) compressing time division multiplexedtraffic by removing idle timeslot data from said time divisionmultiplexed traffic; and, b) adding signalling data to said timedivision multiplexed traffic regarding which idle timeslot data has beenremoved, wherein said signalling data is added to free bits or bitshaving prefixed values in a timeslot 0 of a time division multiplexframe.
 13. The method according to claim 12, further comprising the stepof encapsulating compressed time division multiplex frames into datapackets and forwarding the data packets over the packet switchednetwork.
 14. The method according to claim 12, wherein the packetswitched network type is selected from the group consisting of: InternetProtocol; Multi Protocol Label Switching; Asynchronous Transfer Mode;and, Frame relay.
 15. A method for receiving time division multiplexedtraffic over packet switched networks, comprising the steps of:examining received signalling data in time slot 0 of a time divisionmultiplex frame, said signalling date identifying where idle timeslotdata has been removed; and, decompressing said time division multiplexedtraffic, wherein the time division multiplexed traffic is encapsulatedin data packets, by inserting prefixed idle pattern data into receiveddata packets as a function of said received signalling data.
 16. Themethod according to claim 15, further comprising the step ofdecapsulating the decompressed packet switched traffic into timedivision multiplex traffic.
 17. The method according to claim 15,wherein the packet switched network type is selected from the groupconsisting of: Internet Protocol; Multi Protocol Label Switching;Asynchronous Transfer Mode; and, Frame relay.